The present invention concerns the elimination of mercury and, if present, arsenic from hydrocarbons using a catalyst for transforming mercury containing compounds into elemental mercury and a mercury retention bed, said process resulting in a much higher mercury retention efficiency from the initial startup period of the process.
Liquid condensates (by-products of gas production) and some crude oils are known to contain numerous metallic trace compounds often in the form of organometallic complexes. These metallic compounds very often poison the catalysts used during transformation of these fractions into industrial products.
Purification of these feedstocks for use in transformation processes for condensates or crudes is thus advantageous in order to avoid mercury and possibly arsenic entrainment. Purification of the feedstock upstream of the treatment processes protects the installation assembly.
Some of the assignee's processes perform well as regards mercury removal from liquid hydrocarbons which act as feedstock for various treatment processes. The assignee's U.S. Pat. No. 4,911,825 clearly demonstrates the advantage of mercury retention in a two step process wherein the first step consists in bringing the feedstock in the presence of hydrogen into contact with a catalyst containing at least one metal from the group constituted by nickel, cobalt, iron and palladium. Mercury is not (or is only slightly) retained by the catalyst but it is retained, in a second step, by a bed containing sulphur or a metal sulphide, the metal being copper, iron or silver.
The assignee's patent application WO 90/10 684 describes a process for elimination of mercury from liquid hydrocarbons. This invention concerns catalysts having the ability to resist sulphur poisoning (thioresistance). These novel catalysts allow mercury retention under conditions which are too severe for the catalysts described in the prior art. Thus, this process is particularly useful for purification of difficult feedstocks such as, for example, gas oils from fractionation of crude oil whose sulphur content is frequently between 0.4 and 1.0% by weight.
The process described in U.S. Pat. No. 4,911,825 on the other hand is more effective for feedstocks having a lower sulphur content, for example less than 0.15% by weight.
It has been established, however, that with some feedstocks having a low sulphur content, for example less than 0.07% by weight, the mercury retention efficiency at the beginning of the mercury removal process is lower in the first hundreds of hours; then it rises. It has also been found that the mercury retention efficiency is lower for feedstocks with a very low sulphur content, for example less than 0.02% by weight. In this latter case, it is necessary to increase the operating temperature of the reactor by several dozen degrees and/or increase the hydrogen flowrate to retain sufficient mercury.